1. Field of the Invention
The invention relates to a method for drying lumber by using supercritical fluid, a method for impregnating lumber with chemicals, and a drying apparatus.
2. Description of the Related Art
Pieces of lumber newly cut from trees (green lumber) contain a substantial amount of moisture. The amount of moisture depends on such factors as the type of trees and growth conditions, and often reaches or exceeds one half of green lumber by weight. Because of this, if green lumber is used as housing materials or the like without being dried, the lumber will shrink causing cracking or deformation due to gradual evaporation of moisture after the buildings are completed. In the worst case, this may even result in life-threatening dangerous buildings such as so-called defective home. To avoid such problems, it is necessary to dry lumber by an appropriate amount of moisture before use. Various lumber drying techniques have been used for this purpose.
Air drying, a classic technique for drying lumber, involves stacking pieces of lumber in a staggered fashion to allow water evaporation. This does not require active use of energy but the drying takes a long time, in the order of several months. For this reason, kiln driers are now typically used to complete drying in seven to nine days or so. For a further reduction in the drying period, superheated steam can be used with pressure control, so that humidity is lowered gradually to finish drying in three to four days. Reduced-pressure drying, involving lowering the boiling point by decompression for faster drying, and high-frequency drying for accelerated drying within the lumber as well as at the surface, are sometimes used in combination. A plurality of drying techniques may also be combined as appropriate for a reduced period of treatment and for a uniform finish, though with a considerable increase in cost due to factors such as the amount of energy used.